I. Field of the Invention
These invention relates to improvements in a process for the conversion of methanol to hydrocarbons, to improvements in a Fischer-Tropsch process, and to improvements in Fischer-Tropsch catalysts. In particular, it relates to improved cobalt catalysts, and process for using such catalysts in the conversion of methanol, and Fischer-Tropsch synthesis to produce hydrocarbons, especially C.sub.10+ distillate fuels, and other valuable products.
II. The Prior Art
Methane is often available in large quantities from process streams either as an undesirable by-product in admixture with other gases, or as an off gas component of a process unit, or units. More importantly, however, methane is the principle component of natural gas, and it is produced in considerable quantities in oil and gas fields. The existence of large methane, natural gas reserves coupled with the need to produce premium grade transportation fuels, particularly middle distillate fuels, creates a large incentive for the development of a new gas-to-liquids process. Conventional technology, however, is not entirely adequate for such purpose. Nonetheless, technology is available for the conversion of natural gas, to produce methanol, a prodct of currently limited market ability. However, to utilize the existing technology, there is a need for a process suitable for the conversion of methanol to high quality transportation fuels, particularly middle distillate fuels. On the other hand, the technology to convert natural gas, or methane, to synthesis gas is well established, and the conversion of the synthesis gas to hydrocarbons can be carried out via Fischer-Tropsch synthesis.
Fisher-Tropsch synthesis for the production of hydrocarbons from carbon monoxide and hydrogen is now well known in the technical and patent literature. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100 Co:18 ThO.sub.2 :100 kieselguhr, parts by weight, and over the next few years by catalysts constituted to 100 Co:18 ThO.sub.2 :200 kieselguhr and 100 Co:5 ThO.sub.2 :8 MgO:200 kieselguhr, respectively. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsh reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates. Most commercial experience has been based on cobalt and iron catalysts. The cobalt catalysts, however, are of generally low activity necessitating a multiple staged process, as well as low synthesis gas throughput. The iron catalysts, on the other hand, are not really suitable for natural gas conversion due to the high degree of water gas shift activity possessed by iron catalysts. Thus, more of the synthesis gas is converted to carbon dioxide in accordance with the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.x +CO.sub.2 S; with too little of the synthesis gas being converted to hydrocarbons and water as in the more desirable reaction, represented by the equation: 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.x +H.sub.2 O.
There exists a need in the art for a process useful for the conversion of methanol and synthesis gas at high conversion levels, and at high yields to premium grade transportation fuels, especially C.sub.10 + distillate fuels: particularly without the production of excessive amounts of carbon dioxide.
III. Objects
It is, accordingly, a primary objective of the present invention to supply these needs.
A particular object is to provide novel catalysts, and process for the conversion of methanol and synthesis gas, i.e., carbon monoxide and hydrogen, respectively, to high quality distillate fuels characterized generally as admixtures of C.sub.10 + linear paraffins and olefins.
A further and more specific objective is to provide new and improved supported cobalt catalysts, notably cobalt-titania and cobalt-thoria-titania catalysts, which in methanol conversion reactions, and in Fischer-Tropsch synthesis and subsequent catalyst regeneration, are highly active, and exhibit high stability.
A yet further object is to provide a process which utilizes such catalysts for the preparation of hydrocarbons, notably high quality middle distillate fuels characterized generally as admixtures of linear paraffins and olefins, from methanol, or from a feed mixture of carbon monoxide and hydrogen via the use of such catalysts.
IV. The Invention
These objects and others are achieved in accordance with the present invention which, in general, embodies:
(A) A particulate catalyst composition constituted of a catalytically active amount of cobalt, or cobalt and thoria, to which is added sufficient rhenium to obtain, at corresponding process conditions, improved activity and stability in the production of hydrocarbons from methanol, or in the production of hydrocarbons via carbon monoxidehydrogen synthesis reactions than a catalyst composition otherwise similar except that it does not contain rhenium. Suitably, rhenium is added to the cobalt catalyst, or cobalt and thoria catalyst, in amount sufficient to form a catalyst having a rhenium:cobalt in weight ratio greater than about 0.010:1, preferably from about 0.025:1 to about 0.10:1. In terms of absolute concentrations, from about 0.05 percent to about 3 percent of rhenium, preferably from about 0.15 percent to about 1 percent of rhenium, based on the total weight of the catalyst composition (dry basis), is dispersed with the catalytically active amount of cobalt upon an inorganic oxide support, preferably upon titania (TiO.sub.2), or a titania-containing support, particularly titania wherein the rutile:anatase weight ratio is at least about 2:3. This ratio is determined in accordance with ASTM D 3720-78: Standard Test Method for Ratio of Anatase to Rutile in Titanium Dioxide pigments By Use of X-Ray Diffraction. Suitably, in terms of absolute concentrations the cobalt is present in the composition in amounts ranging from about 2 percent to about 25 percent, preferably from about 5 percent to about 15 percent, based on the total weight of the catalyst composition (dry basis), and sufficient rhenium is added to form a catalyst having a weight ratio of rhenium:cobalt greater than about 0.010:1, preferably from about 0.025:1 to about 0.10:1, based on the total weight of the cobalt and rhenium contained in the catalyst composition (dry basis). The absolute concentration of each metal is, of course, preselected to provide the desired ratio of rhenium:cobalt, as heretofore expressed. Thoria can also be added to the composition, and is preferably added to the catalyst when it is to be used in the conversion of methanol. The thoria is dispersed on the support in amounts ranging from about 0.1 percent to about 10 percent, preferably from about 0.5 percent to about 5 percent, based on the total weight of the catalyst composition (dry basis). Suitably, the thoria promoted cobalt catalyst contains Co and ThO.sub.2 in ratio of Co:ThO.sub.2 ranging from about 20:1 to about 1:1, preferably from about 15:1 to about 2:1, based on the weight of the total amount of Co and ThO2 contained on the catalyst. These catalyst compositions, it has been found, produce a product which is predominately C.sub.10 + linear paraffins and olefins, with very little oxygenates. These catalysts provide high selectivity, high activity, and activity maintenance in methanol conversion, or in the conversion of the carbon monoxide and hydrogen to distillate fuels. These catalysts are also highly stable, particularly during high temperature air regenerations which further extend catalyst life.
(B) A process wherein the particulate catalyst composition of (A), supra, is formed into a bed, and the bed of catalyst contacted at reaction conditions with a mehtanol feed, or feed comprised of an admixture of carbon monoxide and hydrogen, or compound decomposable in situ within the bed to generate carbon monoxide and hydrogen, to produce a product of middle distillate fuel quality constituted precominately of linear paraffins and olefins, particularly C.sub.10 + linear paraffins and olefins.
(i) In conducting the methanol reaction the partial pressure of methanol within the reaction mixture is generally maintained above about 100 pounds per square inch absolute (psia), and preferably above about 200 psia. It is preferable to add hydrogen with the methanol. Suitably methanol, and hydrogen, are employed in molar ratio of CH.sub.3 H:H.sub.2 above about 4:1, and preferably above 8:1, to increase the concentration of C.sub.10 + hydrocarbons in the product. Suitably, the CH.sub.3 OH:H.sub.2 molar ratio, where hydrogen is employed, ranges from about 4:1 to about 60:1, and preferably the methanol and hydrogen are employed in molar ratio ranging from about 8:1 to about 30:1. Inlet hydrogen partial pressures preferably range below about 80 psia, and more preferably below about 40 psia; inlet hydrogen partial pressures preferably ranging from about 5 psia to about 80 psia, and more preferably from about 10 psia to about 40 psia. In general, the reaction is carried out at liquid hourly space velocities ranging from about 0.1 hr.sup.-1 to about 10 hr.sup.-1, preferably from about 0.2 hr.sup.-1 to about 2 hr.sup.-1, and at temperatures ranging from about 150.degree. C. to about 350.degree. C., preferably from about 180.degree. C. to about 250.degree. C. Methanol partial pressures preferably range from about 100 psia to about 1000 psia, more preferably from about 200 psia to about 700 psia.
(ii) In general, the synthesis reaction is carried out at an H.sub.2 :CO mole ratio of greater than about 0.5, and preferably the H.sub.2 :CO mole ratio ranges from about 0.1 to about 10, more preferably from about 0.5 to about 4, at gas hourly space velocities ranging from about 100 V/Hr/V to about 5000 V/Hr/V, preferably from about 300 V/Hr/V to about 1500 V/Hr/V, at temperatures ranging from about 160.degree. C. to about 290.degree. C., preferably from about 190.degree. C. to about 260.degree. C., and pressures above about 80 psig, preferably ranging from about 80 psig to about 600 psig, more preferably from about 140 psig to about 400 psig. In its most preferred form, a bed of catalyst comprised of from about 5 percent to about 15 percent cobalt, containing sufficient rhenium to provide a catalyst containing rhenium:cobalt in ratio ranging from about 0.025:1 to about 0.10:1, is dispersed on titania, preferably a high purity titania, and a bed of such catalyst is contacted with a gaseous admixture of carbon monoxide and hydrogen, or compound decomposable in situ within the bed to generate carbon monoxide and hydrogen.
The product of either the methanol conversion reaction, or synthesis reaction generally and preferably contains 60 percent, more preferably 75 percent or greater, C.sub.10 + liquid hydrocarbons which boil above 160.degree. C. (320.degree. F.).
It is found that cobalt and rhenium, or cobalt, thoria and rhenium, supported on titania, or other titania-containing support provides a catalyst system which exhibits superior methanol conversion, or hydrocarbon synthesis characteristics in Fischer-Tropsch reactions. The titania-containing supports used in the practice of this invention are preferably oxides having surface areas of from about 1 to about 120 m.sup.2 g.sup.-1, preferably from about 10 to about 60 m.sup.2 g.sup.-1.
Rhenium-cobalt/titania and rhenium-thoria- cobalt/titania catalysts exhibit high selectivity in the conversion of methanol to hydrocarbon liquids, or synthesis of hydrocarbon liquids from carbon monoxide and hydrogen. The catalysts employed in the practice of this invention may be prepared by techniques known in the art for the preparation of other catalysts. The catalyst can, e.g., be prepared by gellation, or cogellation techniques. Suitably however the metals can be deposited on a previously pilled, pelleted, beaded, extruded, or sieved support material by the impregnation method. In preparing catalysts, the metals are deposited from solution on the support in preselected amounts to proivde the desired absolute amounts, and weight ratio of the respective metals, or cobalt, rhenium, and thoria. Suitably, the cobalt and rhenium are composited with the support by contacting the support with a solution of a cobalt-containing compound, or salt, or a rhenium-containing compound, or salt, e.g., a nitrate, carbonate or the like. The thoria, where thoria is to be added, can then be composited with the support as a thorium compound or salt in similar manner, or the thorium can first be impregnated upon the support, followed by impregnation of the cobalt, or rhenium, or both. Optionally, the thorium and cobalt, or thoria, cobalt, and rhenium can be co-impregnated upon the support. The cobalt, rhenium and thorium compounds used in the impregnation can be any organometallic or inorganic compounds which decompose to give cobalt, rhenium, and thorium oxides upon calcination, such as a cobalt, rhenium, or thorium nitrate, acetate, acetylacetonate, naphthenate, carbonyl, or the like. The amount of impregnation solution used should be sufficient to completely immerse the carrier, usually within the range from about 1 to 20 times of the carrier by volume, depending on the metal, or metals, concentration in the impregnation solution. The impregnation treatment can be carried out under a wide range of conditions including ambient or elevated temperatures. Metal components other than rhenium and cobalt (or rhenium, cobalt and thorium) can also be added. The introduction of an additional metal, or metals, into the catalyst can be carried out by any method and at any time of the catalyst preparation, for example, prior to, following or simultaneously with the impregnation of the support with the cobalt and rhenium components. In the usual operation, the additional component is introduced simultaneously with the incorporaton of the cobalt and rhenium, or cobalt, rhenium, and thorium components.
Titania is used as a support, or in combination with other materials for forming a support. The titania used for support in either methanol or syngas conversions, however, is preferably one where the rutile:anatase ratio is at least about 2:3 as determined by x-ray diffraction (ASTM D 3720-78). Preferably, the titania used for the catalyst support of catalysts used in syngas conversion is one wherein the rutile:anatase ratio is at least about 3:2. Suitably the titania used for syngas conversions is one containing a rutile:anatase ratio of from about 3:2 to about 100:1, or higher, preferably from about 4:1 to about 100:1, or higher. A preferred, and more selective catalyst for use in methanol conversion reactions is one containing titania wherein the rutile:anatase ranges from about 2:3 to about 3:2. The surface area of such forms of titania are less than about 50 m.sup.2 /g. This weight of rutile provides generally optimum activity, and C.sub.10 + hydrocarbon selectivity without significant gas and CO.sub.2 make.
The catalyst, after impregnation, is dried by heating at a temperature above about 30.degree. C., preferably between 30.degree. C. and 125.degree. C., in the presence of nitrogen or oxygen, or both, or air, in a gas stream or under vacuum. It is necessary to activate the cobalt-titania, or thoria promoted cobalt-titania catalyst prior to use. Preferably, the catalyst is contacted with oxygen, air, or other oxygen-containing gas at temperature sufficient to oxidize the cobalt, and convert the cobalt to Co.sub.3 O.sub.4. Temperatures ranging above about 150.degree. C., and preferably above about 200.degree. C. are satisfactory to convert the cobalt to the oxide, but temperatures up to about 500.degree. C. such as might be used in the regeneration of a severely deactivated catalyst, can generally be tolerated. Suitably, the oxidation of the cobalt is achieved at temperatures ranging from about 150.degree. C. to about 300.degree. C. The cobalt, or cobalt and rhenium metals contained on the catalyst are then reduced. Reduction is performed by contact of the catalyst, whether or not previously oxidized, with a reducing gas, suitably with hydrogen or a hydrogen-containing gas stream at temperatures, above about 250.degree. C.; preferably above about 300.degree. C. Suitably, the catalyst is reduced at temperatures ranging from about 250.degree. C. to about 500.degree. C., and preferably from about 300.degree. C. to about 450.degree. C., for periods ranging from about 0.5 to about 24 hours at pressures ranging from ambient to about 40 atmospheres. Hydrogen, or a gas contaning hydrogen and inert components in admixture is satisfactory for use in carrying out the reduction.
If it is necessary to remove coke from the catalyst, the catalyst can be contacted with a dilute oxygen-containing gas and the coke burned from the catalyst at controlled temperature below the sintering temperature of the catalyst. The temperature of the burn is controlled by controlling the oxygen concentration and inlet gas temperature, this taking into consideration the amount of coke to be removed and the time desired to complete the burn. Generally, the catalyst is treated with a gas having an oxygen partial pressure of at least about 0.1 psi, and preferably in the range of from about 0.3 psi to about 2.0 psi to provide a temperature ranging from about 300.degree. C. to about 50.degree. C., at static or dynamic conditions, preferably the latter, for a time sufficient to remove the coke deposits. Coke burn-off can be accomplished by first introducing only enough oxygen to initiate the burn while maintaining a temperature on the low side of this range, and gradually increasing the temperature as the flame front is advanced by additional oxygen injection until the temperature has reached optimum. Most of the coke can be readily removed in this way. The catalyst is then reactivated, reduced, and made ready for use by treatment with hydrogen or hydrogen containing gas as with a fresh catalyst.
The invention will be more fully understood by reference to the following demonstrations and examples which present comparative data illustrating its more salient features. All parts are given in terms of weight except as otherwise specified. Feed compositions are expressed as molar ratios of the components.
The "Schulz-Flory Alpha" is a known method for describing the product distribution in Fischer-Tropsch synthesis reactions. The Schulz-Flory Alpha is the ratio of the rate of chain propagation to the rate of propagation plus termination, and is described from the plot of 1 n (Wn/n) versus n, where Wn is the weight fraction of product with a carbon number of n. In the examples below, an Alpha value was derived from the C.sub.10 /C.sub.20 portion of the product. The Alpha value is thus indicative of the selectivity of the catalyst for producing heavy hydrocarbons from the synthesis gas, and is indicative of the approximate amount of C.sub.10+ hydrocarbons in the product. For example, a Schulz-Flory Alpha of 0.80 corresponds to about 35% by weight of C.sub.10+ hydrocarbons in the product, a generally acceptable level of C.sub.10+ hydrocarbons. A Schulz-Flory Alpha of 0.85, a preferred Alpha value, corresponds to about 54% by weight of C.sub.10+ hydrocarbons in the products, and a Schulz-Flory Alpha of 0.90, a more preferred Alpha value, corresponds to about 74% by weight of C.sub.10+ hydrocarbons in the product.